The invention is based on the use of metallocene catalyzed polyolefin elastomer grafted with maleic anhydride (MAH) in hot melt adhesives for applications that require increased hot tack, adhesion, flexibility, and heat resistance above 150° F.
Hot melt adhesives are used in packaging applications where it is often required to adhere to low porosity paper or cardboard which are covered with a wide range of inks, coatings, and overprint lacquers for functionality or aesthetic purposes. In other instances they are used to bond low surface energy substrates such as polyethylene (PE) or polypropylene (PP) films, and to boxes or substrates that vary in physical characteristics such as the rigidity, density, and chemical make-up due to high level of recycled material. These type of substrates are very difficult to bond to. As a result, adhesives must be formulated to overcome these obstacles. Consequently, adhesive formulators are continuously evaluating new materials and novel formulation strategies in order to develop an adhesive with the broadest possible application window. An adhesive's application window is defined as an adhesive's ability to overcome an application's deficiencies and/or manufacturing variables. The current invention details a novel way a hot melt formulator can balance the adhesive's application window without adversely affecting the high temperature environmental resistance and adhesion to the substrates.
Historically, adhesive formulators have struggled to formulate a crystalline, polyolefin based adhesive or an EVA based adhesive that could provide heat environmental resistance above 150° F. while maintaining the hot tack and/or cold temperature performance. In order to increase the heat resistance over 150° F., typically a styrenic block copolymer based polymer is used. In particular they have fully hydrogenated midblocks, such as Kraton G1657. They can be used to increase the heat resistance properties of the adhesive while maintaining adequate compatibility with the polyolefin base polymer. Another approach is to use a high level of polymer (for example, greater than 30%) to increase the heat resistance while maintaining the cold temperature resistance. However, these approaches drive the viscosity of the adhesive significantly higher and it becomes unsuitable for the intended applications.
Adhesives have been used for years to label both glass and plastic bottles. Plastic bottles containing carbonated beverages are particularly challenging. After bottling, carbonated beverages will cause plastic bottles to expand. The label system needs to accommodate this expansion. The adhesive used to adhere the label onto the plastic bottle also needs to accommodate this expansion.
Paper labels are rigid and will not expand. Typically, hard, glassy adhesives are used to bond the paper label to glass bottles. When paper labels are used on plastic bottles, the adhesive needs to resist creep, and thus prevent the label from “flagging” (partial delamination of the label overlap from the bottle). However, plastic labels are flexible and will expand and are generally more difficult to adhere to than the paper labels. Typically, softer, elastic, tacky adhesives are used on plastic film labels. The adhesive used to adhere plastic labels needs to be of sufficient tack to adhere the label to the bottle, and strong enough to withstand the expansion of the label and the bottle. In this type of application, the adhesive needs to have a greater internal strength than that of the label. The adhesives' increased internal strength forces the label to stretch and expand, maintaining the bond at the label overlap. Should the adhesive stretch or creep a gap will appear between the leading and trailing edge of the label on the bottle at the label overlap.
Plastic labels are becoming more rigid in order to support increased graphics and facilitate the printing process. Additionally, plastic bottles are experiencing an overall gauge reduction to achieve cost savings and meet manufacturers' “green” initiatives. Also, clear labels are also entering the market place. These labels, due to their base composition, have greater tensile strength than previous plastic labels. The greater tensile strength of these labels resists stretching, causing existing adhesives to creep, resulting in a labeling failure (the label's leading and trailing edges separate). Furthermore, some plastic film labels are susceptible to oil migration from the adhesive into the label. This migration causes aesthetically unpleasing wrinkles.
Adhesive formulators are struggling to develop adhesives that adhere well to these higher tensile strength plastic labels, and do not exhibit oil migration from the adhesive into the label at 140° F. This invention details a novel way a hot melt formulator can impart low viscosity, superior adhesion, increased creep resistance, and no oil migration in hot melt bottle labeling adhesives, without compromising other properties.
There has been a need for hot melt adhesives with increased temperature resistance for “hot fill” applications. This is where a liquid food product (juice, tea, etc.) is heated to a temperature of about 190° F. to sterilize it. The liquid is placed in a plastic or glass container while hot which serves to sterilize the package as well. It is immediately capped, which helps provide a vacuum as the liquid cools. Once capped it is quickly cooled to minimize the effect of heat on the liquid. The label may be applied before the container is completely cooled and therefore requires increased temperature resistance so that the label does not detach from the bottle.
There is also a need for “hot fill” applications involving carton sealing. Besides hot fill label applications, there are other times when hot materials are put into a shipping container while still hot. This can cause the hot melt adhesive to soften to the point that the bond delaminates. Clearly, there is a need for hot melt adhesives with better heat resistance in a number of areas.
Over the years, adhesive formulators have utilized a variety of different polymers as well as other additives in their formulations to obtain a balance of these attributes (adhesion, creep resistance, flexibility, and heat environmental resistance). These polymers include, but are not limited to polyolefins (ethylene- or propene-based polymers), functionalized polyolefins (ethylene or propene copolymers with oxygen containing monomers), or APAOs (ethylene-, propene-, or butene copolymers). However, when formulated into hot melt adhesives, these polymers had certain performance deficiencies. For example, due to their overall wide molecular weight distribution and/or significant low molecular weight portion as observed by various analytical testing methods, APAOs can provide flexibility but can hamper bonding performance at elevated temperatures above 120° F. In fact, their amorphous, non-crystalline structure can often lead to blocking. Blocking is defined as the undesired adhesion of a coated adhesive to substrates it comes into contact with during shipping and/or storage.
In addition to ethylene vinyl acetate (EVA) polymers other polymers have also been utilized in an attempt to improve an adhesive's hot tack and adhesion characteristics. These polymers include, but are not limited to ethylene methyl acrylate copolymers (EMA), ethylene n-butyl acrylate (EnBA), and ethylene methyl acrylate acrylic acid copolymers. These polymers exhibit narrower poly-dispersity when compared to olefin polymers, such as APAO and have lower overall melt peaks as observed by DSC (Differential Scanning calorimetry). This results in an adhesive that is prone to blocking or bond failure at elevated temperatures if not reinforced with some other crystalline additive. While the incorporation of certain waxes or other crystalline additives can increase the elevated temperature resistance of the adhesive, they can reduce the adhesive's hot tack, adhesion, and flexibility.
Adhesive formulators may incorporate other additives or diluents including but not limited to various plasticizers, microcrystalline waxes, and vinyl acetate or maleic anhydride modified waxes to promote adhesion and flexibility. However, these types of formulations typically have insufficient heat resistance above 150° F.